Rare earth nitrate solutions of commerce usually consist principally of the nitrates of lanthanum, cerium, praseodymium and neodymium, sometimes with minor proportions of the nitrates of other rare earth elements. The proportions of the principal rare earth elements present may vary, depending on the method of manufacture and on the rare earth-containing mineral used for their extraction. Most commercial products contain a substantial proportion of cerium, commonly 45% of the total rare earth metals.
Such solutions of rare earth nitrates are generally made by dissolving rare earth hydrates (or hydroxides) in nitric acid. In the manufacture of these hydrates some or all of the cerium becomes oxidized from the trivalent to the quadrivalent state since cerous hydroxide is readily oxidized, even by exposure to the atmosphere, to ceric hydroxide or hydrated ceric oxide. This presents difficulties in preparing nitrate solutions since, although all the rare earths, including cerium in trivalent form, readily dissolve as hydrates in nitric acid to produce solutions of pH above 2 and up to 3.5, when cerium is present in the hydrates in quadrivalent form it will only dissolve in nitric acid if the pH of the solution is maintained below 0.7. A solution of this degree of acidity is undesirable for some applications, such as for making rare earth-modified zeolitic fluid cracking catalysts. Moreover, nitric acid solutions containing quadrivalent cerium are highly corrosive to stainless steel which is one of the most commonly used constructional materials for chemical process plants. It is desirable, therefore, to reduce any quadrivalent cerium to trivalent cerium, which presents no such limitations on solution pH and no abnormal corrosion properties.
Many reducing agents for quadrivalent cerium in acid solution are known. Examples include sucrose, formaldehyde, sulfur dioxide, hyrazine and hydroxylamine. However, all of these have significant disadvantages. Sucrose and formaldehyde react with nitric acid, evolving toxic oxides of nitrogen, and these materials often exhibit a delayed reaction which may be violent. Hydrazine and hydroxylamine both form nitrates which, if dry (as may be the case if the solution is subsequently concentrated by evaporation), may be explosive. Sulfur dioxide produces sulfate ions which are undesirable in rare earth nitrate solutions used for catalyst manufacture.
It is therefore an objective of the present invention to provide a process for the preparation of a rare earth nitrate solution of pH greater than 1 by dissolution in nitric acid of rare earth hydrates containing cerium in quadrivalent form without the objectional features associated with conventional reducing agents. It is another objective of the invention to provide a process control mechanism for the preparation of such solutions.